In a digital wireless transmission and reception system, a transmitter transmits a modulated signal comprising a carrier modulated with digital data having “1” or “0” level, while a receiver receives the modulated signal and recovers the digital data from the received signal. Being affected by noise or the like in the radio transmission path between the transmitter and the receiver, it may occur that the “1” or “0” level data cannot properly be recovered to have “1” or “0” level.
One technique for examining whether or not there is any such error is the RSSI system. According to this system, the reception level is examined at a receiver to find out portions of digital data which may contain error. Another technique is to add, at a transmitter, an error detection code to each sample or each frame of digital data before transmission and judge whether or not erroneous transmission has been done, based on the error detection code. One sample is, when an audio signal, for example, is transmitted, a digital signal with a predetermined number of bits which results from sampling the audio signal at a predetermined sampling frequency and converting each of the resulting sampling values in an analog-to-digital (A/D) converter. One frame is a plurality of such digital signals combined into a lump of data. Also, a technique has been developed, according to which an error detection code is added at the transmitter and error correction is done at the receiver.
However, the RSSI system can only approximately specify portions which may contain error.
When error detection codes are used to detect error, an error detection code is added to each sample or frame, and, therefore, if error is detected in an sample or frame, such sample or frame must be discarded. Even when portions of a sample or frame containing error are few, the whole sample or frame of data must be discarded. Even if only one sample or frame is discarded, particularly in a digitized audio signal, the sound is distorted large relative to small error in data.
A typical example of error correction technique is the BCH code technique, according to which redundant bits are added to information bits so as to make it possible to make correction to an extent determined by the correction ability. For example, by adding three redundant bits to four information bits, the correction ability of one bit is provided. In this case, if error exceeding the correction ability occurs, inappropriate correction is done, which results in large distortion. If it is desired to increase the correction ability in order to avoid inappropriate correction, the number of redundant bits must be increases, which results in decrease of transmission efficiency. In particular, in wireless communication signals, error in successive bits, called burst error, may occur. Accordingly, in order to provide error correction by means of error correction codes, the correction ability must be increased relative to random error of the same error rate. As a result, more redundant bits must be added, which results in further reduction of transmission efficiency.
An object of the present invention is to provide a transmitter and receiver, which enables highly accurate error detection and reliable code error correction without introducing reduction of transmission efficiency.